Most UPS's pass line voltage straight through unless it disappears or goes out of range. It's more efficient and results in less wear and tear than running Ac->DC->AC all the time. You can usually tell when they are generating power by the neat buzzing noises they make. So they all have an ocsillator for when they need to generate power, but otherwise what goes in is what comes out (voltage and especially frequency). A 240V/50Hz ups will probably generate the desired 240V/50Hz signal only when in battery only mode (or plugged into a 50Hz line source). Check the specs to be sure. KL Sean Donelan wrote:
Actually, its not being clever, its being cheap. If you use the input frequency to drive the output frequency, you don't need an extra crystal oscillator. Its cheaper, and results in fewer customer service calls about the clock running fast or slow. It also means, you can't just take a 50/Hz european UPS and plug it into an US plug at 220vac, and expect to still get 50/Hz out of it. When you plug it into the US grid, you'll get 60/Hz out.